1. Field of the Invention
The present invention relates to railway air system air dryers and, more particularly, to an air dryer having a heating control system for preventing freezing of valves.
2. Description of the Related Art
A typical “twin-tower” desiccant-type air dryer includes two drying circuits that are controlled by valves. Wet inlet air flows through one circuit to remove water vapor, while dry product air counter flows through the other circuit to remove the accumulated water and regenerate the desiccant. Inlet and exhaust valves for each pneumatic circuit are responsive to controlling electronics to switch the air flow between the two circuits so that one circuit is always drying while the other is regenerating. For example, when inlet A is open, exhaust A is closed, inlet B is closed, and exhaust B is open for the duration of the regeneration cycle, but otherwise closed. Likewise, when inlet B is open, exhaust B is closed, inlet A is closed, and exhaust A is open for the duration of the regeneration cycle, but otherwise closed.
The air dryer valves are constantly subjected to wet air and thus prone to freezing at low temperatures. Because the two sets of inlet and exhaust valves are independently controlled, it is possible for an inlet valve and an exhaust valve in the same circuit to be simultaneously open if either or both of the inlet and exhaust valves freeze open. When the inlet and exhaust valves are simultaneously open, the main locomotive reservoir will vent and the locomotive air compressor will have insufficient capacity to overcome the air loss. In a second scenario, although the inlet valves are normally open (no power), it is possible that one or both of these valves might freeze closed. If both inlet valves are simultaneously closed, then all flow through the dryer is blocked.
In order to counteract this problem, a heater element may be provided to warm the valves sufficiently to prevent freezing. At start-up, a controller determines the temperature of the valve block. If the valve block temperature is less than a predetermined temperature, such as 5 degrees C., then the controller turns on the heater and delays powering of any of the two sets of the normally open inlet valves and the normally closed exhaust valves until such time that the valve block temperature is equal to or greater than the predetermined temperature. In a conventional approach, this evaluation only occurs at initial start-up and it is therefore assumed that the heater and controller will maintain the valve block temperature above the predetermined temperature and freezing will not occur. In practice, however, the heater may not have enough power to maintain the valve block above the predetermined temperature when ambient temperatures are very and the air high flow rate is high due to the substantial heat loss to the cold inlet air flow through the dryer. As a result, even though the heater may successfully warm the valve block at a cold temperature start-up, the valve block may drop below freezing during high flow operations so that the inlet and exhaust valves can freeze and resulting in the problems described above. Accordingly, there is a need for a heater control system that can address the problem of conditions that cause freezing of the valves after initial start-up based on subsequent operating condition.